Method for controlling a drive-off process of a railway vehicle

ABSTRACT

A method controls a drive-off process of an electrically driven vehicle, the electric motor of which is fed via a converter. A holding torque necessary to prevent the vehicle from rolling back is determined. By using sensors for determining carriage masses and sensors for determining route inclinations, the holding torque can be precisely determined. In order to achieve a drive-off process of the electrically driven vehicle that is gentle on the electric motor and as long as a determined rotational motor speed is less than a specified limit rotational speed, a traction torque is limited by a control unit of the vehicle to a limit torque dependent on the holding torque, and the traction torque is increased beyond the limit torque by the control unit only once the rotational motor speed is greater than the limit rotational speed.

The invention concerns a method for controlling a drive-off process ofan electrically driven vehicle, with which a holding torque necessaryfor preventing the vehicle from rolling back is determined.

Electric motors, in particular alternating current motors, ofelectrically driven vehicles, are frequently supplied by a converter.The converter produces an output voltage from an input voltage, theoutput voltage being output in the form of pulses with adjustable pulseduration and/or adjustable pulse height to an electric motor of thevehicle. Said output voltage is preferably a three-phase system withvariable frequency and voltage amplitude.

The motor revolution rate of a three-phase motor is—for a predeterminedload characteristic curve—dependent on the frequency and voltage of thepower supply. The motor revolution rate increases in particular withrising frequency of the output voltage. Therefore, the motor revolutionrate can be controlled by regulating the frequency and the voltage ofthe output voltage produced by the converter.

It is the object of the present invention to disclose a method withwhich a machine-friendly drive-off process of an electrically drivenvehicle is enabled.

This object is achieved by a method of the aforementioned type, withwhich, according to the invention, while a determined motor revolutionrate is less than a predetermined first revolution rate limit, atraction torque is limited by a control unit of the vehicle to a torquelimit dependent on the holding torque and the traction torque is onlyincreased by the control unit to above the torque limit if the motorrevolution rate is greater than the first revolution rate limit.

The invention is based on the consideration that with a drive-offprocess of the vehicle a high thermal load on the converter can occur,in particular on a semiconductor component of the converter, which canreduce the service life of the converter.

The invention is further based on the consideration that at lowfrequencies of the output voltage produced by the converter and hencealso at low motor revolution rates, such as occur during a drive-offprocess of a vehicle, a conducting phase of the semiconductor componentof the converter is relatively long. Consequently, the semiconductorcomponent can heat up for a relatively long time in a conducting phaseand can reach a high temperature if the motor revolution rate is low. Amaximum temperature of the semiconductor component that is reachedduring a conducting phase of the semiconductor component can depend onthe traction torque that is set for the respective motor revolutionrate. Said temperature can increase with increasing traction torque.

At a higher frequency of the output voltage produced by the converter,and hence also at a higher motor revolution rate, a conducting phase ofthe semiconductor component is shorter, as a result of which thesemiconductor component heats up for less time in a conducting phase. Asa result, it is possible that a maximum temperature of the semiconductorcomponent that is reached during a conducting phase of the semiconductorcomponent at a higher motor revolution rate adopts a lower value thanfor a low motor revolution rate—even in the case of a higher settraction torque.

That part of the drive torque that is produced by one or more electricmotors of a vehicle that acts in total on wheels of the vehicle and cancontribute to a transfer of traction force to a road surface can beconsidered to be the traction torque.

The vehicle can comprise inter alia a single carriage. It can howeveralso comprise a plurality of carriages that are coupled together. Atleast one carriage of the vehicle comprises an electric drive, whereinthe electric drive comprises at least one electric motor that issupplied by means of a converter.

The vehicle can be for example a railway vehicle or a motor vehicle. Ifthe vehicle is a motor vehicle, then the vehicle can for examplecomprise an electrically driven truck or automobile as well as one or aplurality of carriages without their own drives that are coupled to thetruck or automobile. If the vehicle is a railway vehicle, then thevehicle can comprise one or a plurality of driven carriages and one or aplurality of carriages without their own drives.

The vehicle is fitted with an electric drive that comprises one or aplurality of electric motors. The same are supplied from one or aplurality of converters, wherein each electric motor can be supplied bya converter or a plurality of electric motors can be supplied by oneconverter.

A holding torque can be the minimum traction torque to be applied by theelectric motor in order to prevent rolling back by the vehicle, forexample on an upslope. In general, rolling against a direction of travelspecified by a driver or an automatic control system can be consideredto be rolling back.

The point in time at which the traction torque is increased or a braketorque is reduced after stopping the vehicle—whichever occurs first—isdefined as the start of a drive-off process.

A brake torque can be a torque produced by one or a plurality of brakesystems that acts in total on wheels or wheel axles of the vehicle. Thepurpose of the brake torque can for example be to hold the vehicle at astandstill, in particular on an upslope or a downslope. A brake systemcan comprise one or a plurality of brakes, in particular a brake perwheel or wheel axle of the vehicle. In particular, if the vehicle is arailway vehicle, the vehicle can comprise further brakes that canproduce braking forces directly between the vehicle and the tracksurface. When using said further brakes, the brake torque can be a sumof a torque produced by the further brakes and the torque produced bythe brake system that acts on the wheels or wheel axles of the vehicle.

Before the start of the drive-off process, the traction torque isadvantageously zero, hence unnecessary heating up of the electric motorand/or of the converter can be avoided. Moreover, before the start ofthe drive-off process the brake torque is usefully at least as large asthe holding torque, so that rolling back of the vehicle is prevented.

A control unit can be understood to mean a device that is configured forcontrolling the traction torque. The control unit can in particularcomprise automatic traction control. The control unit can also beconfigured for controlling one or a plurality of brake systems, inparticular for applying and/or releasing one or a plurality of brakes.

A torque limit can be a value of the traction torque to which thetraction torque is limited by the control unit, wherein the value can bedependent inter alia on the determined holding torque. The torque limitis advantageously greater than the holding torque, so that the vehiclecan be accelerated from rest in the direction of travel, in particularwithout rolling back in doing so.

A motor revolution rate is advantageously determined repeatedly, inparticular at fixed time intervals. A motor revolution rate sensor canbe used to determine the motor revolution rate for example. A vehiclespeed can be proportional to the motor revolution rate; in this respectthe speed can be measured additionally or alternatively, wherein forsimplicity a speed measurement is also understood below as determining amotor revolution rate.

A revolution rate limit can be a design-related revolution rate value.The revolution rate limit can in particular depend on the converterdesign. The revolution rate limit can also depend on the torque limit.The revolution rate limit can in particular be greater, the greater isthe torque limit.

The higher a semiconductor component temperature that is reached duringa drive-off process in a conducting phase of a semiconductor componentof the converter, the shorter the semiconductor component service lifecan be. The traction torque is therefore advantageously controlled bythe control unit so that a maximum semiconductor component temperaturereached during a conducting phase of a semiconductor component of theconverter remains below a specified temperature value.

Said feature of the method can for example be implemented by storing asemiconductor component temperature calculated as a function of aplurality of parameters in the control unit as an especiallymulti-dimensional table. A set of possible parameter values can beprovided in the table for each of the parameters on which the calculatedtemperature depends.

The calculated temperature can be a function of: the holding torque, thetraction torque, the brake torque, the motor revolution rate and/ordesign-related converter parameters.

The calculated semiconductor component temperature can be acquired atspecified time intervals using the known and/or determined parametervalues in the table. The calculated temperature can be compared with thespecified temperature value. The torque limit can then be increased orreduced by the control unit.

The temperature of the semiconductor component can for example relate toa temperature on a contact surface with a bonding wire. The bonding wirecan be arranged to connect the semiconductor component electricallyconductively to one or a plurality of components, in particular toconnections of a chip housing that can enclose the semiconductorcomponent. The bonding wire is preferably soldered or welded to thesemiconductor component.

The semiconductor component and the bonding wire can comprise differentmaterials. The bonding wire can for example consist essentially ofaluminum. The semiconductor component preferably consists essentially ofsilicon.

Different, in particular material-dependent coefficients of thermalexpansion of the bonding wire and of the semiconductor component can,following a defined number of switching cycles of the semiconductorcomponent, result in a crack on the contact surface of the bonding wireand as a result in a failure of the converter. The number of switchingcycles following which a failure of the converter can occur can bedependent on: a material of the bonding wire, a material of thesemiconductor component, a geometry of the bonding wire, a geometry ofthe semiconductor component and/or operating parameters of theconverter.

The semiconductor component can in particular be a bipolar transistorwith an insulated gate electrode (insulated-gate bipolar transistor).

In an advantageous embodiment of the invention, the torque limit is lessthan the maximum of 1.3 times the holding torque and 0.3 times themaximum traction value that can be set by the control unit. At thesevalues converter-friendly driving off can be achieved even on a steepupslope or when driving off on a hill.

Furthermore, the torque limit is preferably above the maximum of 1.2times the holding torque and 0.2 times the maximum of the traction valuethat can be set by the control unit. In this way it is achieved that thetorque limit is not too low and, despite the converter protection, rapiddriving off is enabled.

As a result of the torque limit being determined by means of a maximumfunction, it can be achieved that the torque limit can be set by thecontrol unit to a fixed predetermined value for low values of theholding torque. Furthermore, it can be achieved that for high values ofthe holding torque the torque limit is set by the control unit to avalue that is dependent on the holding torque, and that in particular isgreater than the holding torque. In this way, an advantageous compromisebetween converter protection and rapid driving off can be achieved.

In one version of the invention, a band of torques with a maximum valueand a minimum value of the traction torque can be stored in the controlunit. Preferably, the traction torque can only be set within said bandof torques if the determined motor revolution rate is less than thepredetermined first revolution rate limit. In this way it can beprevented that the traction torque is set to a value that is damaging tothe converter by an external intervention. In this case the tractiontorque can be adjusted by the driver or by an external controller forexample.

The maximum value of the band of torques can in particular be themaximum of 1.3 times the holding torque and 0.3 times the maximum of thetraction value that can be set by the control unit. The minimum value ofthe band of torques can in particular be the maximum of 1.2 times theholding torque and 0.2 times the maximum of the traction value that canbe set by the control unit.

If such a band of torques is stored in the control unit, in the absenceof an external intervention, for example by the driver or by theexternal controller, the traction torque can be held constant by thecontrol unit at a predetermined value, as long as the determined motorrevolution rate is less than the first revolution rate limit. Thepredetermined value can in particular be the average of the maximumvalue and the minimum value.

The traction torque is advantageously held by the control unit above aminimum torque, preferably between the minimum torque and the torquelimit, once the traction torque is at least as great as the minimumtorque. In this way it can be achieved that a dwell period of thevehicle in a state with a low motor revolution rate is reduced. It isuseful for the minimum torque to be less than the torque limit.

The minimum torque can be dependent on the holding torque. The minimumtorque is preferably greater than the holding torque. As a result,despite any inaccuracies in determining the holding torque or despitethe effect of further influences, such as for example a headwind orfriction, the vehicle can be prevented from rolling back—even without aholding action of a brake.

The minimum torque can be greater than the holding torque by apredetermined percentage value of the holding torque, for example 10%.If, however, the determined holding torque is less than a specifiedthreshold value, in particular if the holding torque is zero, theminimum torque can be a fixed predetermined value, for example 15% ofthe maximum of the traction value that can be set by the control unit.

A further version of the invention provides that the torque limit and/orthe minimum torque can be set by the driver, in particular in stages.Advantageously, it is possible for the driver to completely cancellimiting of the traction torque, in particular in the presence of anoperational exception situation.

In a preferred development of the invention, once the motor revolutionrate is greater than the first revolution rate limit the traction torqueis increased by the control unit until a predetermined second revolutionrate limit is reached. As a result, a higher speed and/or accelerationof the vehicle can be achieved. At the same time, despite an increase ofthe traction torque, the maximum temperature of the semiconductorcomponent that is reached during a conducting phase of the semiconductorcomponent can be lower than in the period of time in which the motorrevolution rate is less than the first revolution rate limit owing to ashorter duration of the conducting phase.

The traction torque is preferably increased linearly, in particularproportionally, relative to the motor revolution rate, once the motorrevolution rate is greater than the first revolution rate limit and aslong as the motor revolution rate is less than the second revolutionrate limit.

Preferably, on reaching the second revolution rate limit the tractiontorque adopts the maximum of the traction value that can be set by thecontrol unit. After adopting the maximum of the traction value that canbe set by the control unit, the traction torque is preferably heldconstant by the control unit until the maximum motor power is reached atsaid traction value.

The first revolution rate limit is advantageously set by the controlunit depending on the holding torque. As a result, it is possible that arange of revolution rates, within which the traction torque is limitedto protect the converter, can be adapted depending on the situation. Inparticular, the first revolution rate limit can be set by the controlunit to a value that is greater, the greater is the holding torque.

The ratio of the second revolution rate limit to the first revolutionrate limit is preferably equal to the ratio of the maximum of thetraction value that can be set by the control unit to the torque limit.

In an advantageous version of the invention, a gradient parameterdependent on a track slope angle is determined. The track slope angle isusefully related to a track segment on which the vehicle is located. Thetrack slope angle can in particular be a value that is averaged over anentire length of the vehicle.

The gradient parameter can for example be the track slope angle itself.Alternatively, the gradient parameter can be a component of theacceleration due to gravity that is dependent on the track slope angleand that is oriented parallel to the track segment. A component of theacceleration due to gravity that acts as a downhill acceleration on thevehicle can be determined from the gradient parameter in a simplemanner.

A positive value of the gradient parameter can represent a positivetrack slope angle, wherein the positive track slope angle can occur onan upslope. A negative gradient parameter can represent a negative trackslope angle, wherein the negative track slope angle can occur on adownslope. Upslope and downslope are also understood to relate to thedirection of travel of the vehicle.

The gradient parameter can be determined using an accelerometer forexample. In particular, the accelerometer can be an element of aninertial measurement unit that comprises, besides the accelerometer, atleast one further accelerometer and/or at least one rate of turn sensor.In order for example to achieve greater accuracy when determining thegradient parameter, the gradient parameter can be determined using aplurality of accelerometers. In addition, at least one rate of turnsensor can be used when determining the gradient parameter.

It is also advantageous if a mass of the vehicle is determined. If thevehicle comprises a pneumatic suspension system, then the mass can forexample be determined from a measurement of an air pressure. If thevehicle comprises a suspension system with coil springs, then the masscan for example be determined from measurements of axial lengths of thecoil springs.

The holding torque is advantageously calculated from the mass of thevehicle and from the gradient parameter. The control unit isadvantageously set up to calculate the holding torque from the mass andthe gradient parameter.

Advantageously, a respective carriage gradient parameter is determinedfor each carriage of the vehicle. As a result, it can be taken intoaccount that the carriage can be standing on track segments withdifferent track slope angles. In order to be able to determine therespective carriage gradient parameter, each carriage can be fitted withat least one dedicated accelerometer.

A carriage mass is preferably determined for each carriage of thevehicle. A total mass of the vehicle can be calculated from the carriagemasses. Advantageously, the control unit is set up to calculate thetotal mass of the vehicle.

A carriage holding torque is preferably calculated from the respectivecarriage mass and from the respective carriage gradient parameter foreach carriage of the vehicle. The holding torque of the vehicle isadvantageously calculated by summing up all determined carriage holdingtorques.

In an advantageous embodiment version, a substitute value calculation isprovided in the control unit, which refers to substitute values and/orsubstitute algorithms if one or a plurality of individual values islost. Thus, for example, if a carriage gradient parameter is notavailable, for example because of an accelerometer defect, a carriagegradient parameter can be extrapolated or interpolated from one or aplurality of other carriage gradient parameters. If there is a carriagepositioned both in front of and behind the carriage, the carriagegradient parameter of which is not available, then the missing carriagegradient parameter can be set equal to the average of the carriagegradient parameters of said two carriages. If the carriage, the carriagegradient parameter of which is not available, is only adjacent to onecarriage, then the missing carriage gradient parameter can be set equalto the carriage gradient parameter of the adjacent carriage.

It is also advantageous if the substitute value calculation refers tosubstitute values and/or substitute algorithms if one or a plurality ofindividual values is lost during the determination of the carriagemasses. Thus, for example, if the carriage mass of a carriage is notavailable, a maximum mass, in particular a maximum permissible maximummass, of the carriage can be set as the carriage mass.

The respective carriage-holding torque can inter alia be greater thanzero if a component of a weight force of the respective carriage isacting against the vehicle direction of travel, i.e. for example if thecarriage is on an upslope in relation to the direction of travel. Therespective carriage holding torque can inter alia be less than zero if acomponent of the weight force of the respective carriage is acting inthe direction of travel of the vehicle, i.e. for example if the carriageis on a downslope in relation to the direction of travel.

Summing all determined carriage holding torques for the calculation ofthe holding torque of the vehicle usefully takes place while taking intoaccount a sign of the respective carriage holding torque. If thedetermined holding torque is less than zero, it is advantageously set tozero by the control unit.

The gradient parameter is or the carriage gradient parameters areadvantageously determined repeatedly, in particular at fixed timeintervals. The holding torque of the vehicle is usefully calculated fromthe last determined gradient parameter or from the last determinedcarriage gradient parameters. As a result, changes of the gradientparameter or the carriage gradient parameters that occur during thedrive-off process can be taken into account and the current holdingtorque can always be calculated.

Advantageously, a brake is released by the control unit for driving off.The brake torque then reduces from an initial brake torque to zero. Afurther advantageous embodiment of the invention provides that theincrease in the traction torque is carried out as a function of areducing brake torque. The control unit thus controls the increase ofthe traction torque as a function of brake torque, the variation ofwhich with time can be stored in the control unit, for example bystoring the variation against time of a release command as a function ofan initial brake torque. Owing to such synchronization, heating up ofthe converter can be kept low.

The traction torque advantageously increases at least over a timesegment to the extent that the brake torque reduces. The time segmentcomprises here at least half the time required to fully release thebrake.

It is advantageous if the brake is released by the control unit beforethe traction torque is increased by the control unit, in particularstarting at zero. As a result, an unnecessarily long holding action bythe brake can be avoided.

The holding action of the brake is for example unnecessary at the latestfrom the point in time at which the traction torque is as large as thetorque limit, since it is possible that the holding action of the brakefrom said point in time only hinders driving off, but is no longernecessary for preventing the vehicle from rolling back.

It is advantageous if the traction torque is controlled by the controlunit such that the traction torque first reaches the torque limit whenthe brake torque reaches the value zero. As a result, it can beprevented that the brake torque counteracts the traction torque and as aresult hinders driving off when the traction torque reaches the torquelimit.

In a preferred development of the invention, the traction torque isincreased by the control unit such that the sum of the traction torqueand the brake torque remains constant, in particular greater than theholding torque. The sum of the traction torque and the brake torque isusefully constant only from the point in time at which the tractiontorque is increased.

Furthermore, the traction torque can be increased by the control unitsuch that the sum of the traction torque and the brake torque remainsconstant and at least as great as the torque limit, in particular equalto the torque limit.

The sum of the traction torque and the brake torque can in particularrelate to the magnitude of a vector sum, because the traction torque andthe brake torque can act in different directions during the drive-offprocess.

It is also advantageous if the traction torque is increased, inparticular starting from zero, once the brake torque falls below thetorque limit. This makes it possible that the sum of the traction torqueand the brake torque remains at least as great as the torque limitdespite a decrease of the brake torque.

In an advantageous embodiment of the invention, a first point in time atwhich the brake torque has fallen to zero is pre-calculated. A precedingsecond point in time can be calculated from the first point in time. Thetraction torque at the second point in time, in particular starting fromzero, preferably reaches the torque limit at the first point in time byrising at the maximum allowed rate. This allows a period of time ofcounteracting traction torque and brake torque to be reduced.

The maximum allowed rate at which the traction torque is increased canbe lower than a technically maximum possible rate at which the tractiontorque can be increased. The maximum allowed rate can be a limited ratefor reasons of passenger comfort, in particular in relation to avoidingsudden jolts, and/or to protect a drive train of the vehicle.

If the motor revolution rate is already greater than the firstrevolution rate limit and if the motor revolution rate, for exampleowing to braking, has fallen below the first revolution rate limit, thenit is advantageous if the traction torque is limited by the control unitto the torque limit, as long as the motor revolution rate is less thanthe first revolution rate limit. In particular, if the motor revolutionrate falls below the first revolution rate limit the traction torque canbe held at the torque limit by the control unit as long as the motorrevolution rate is less than the first revolution rate limit. It is alsoadvantageous if the traction torque is increased by the control unit toabove the torque limit once the motor revolution rate is again greaterthan the first revolution rate limit.

In a further advantageous version of the invention, the traction torqueis a negative traction torque that is preferably used to brake thevehicle electrically. In relation to the advantageous developments ofthe invention described above, a magnitude of the negative tractiontorque is decisive for controlling the traction torque by the controlunit. As a result, converter-friendly electrical braking can beachieved.

It is advantageous if control of the traction torque by the control unitcan be deactivated by the driver during electrical braking orautomatically deactivated in the event of emergency braking, so thatrapid deceleration of the vehicle is possible, in particular to astandstill.

The invention further concerns a control system for an electricallydriven vehicle with at least one electric motor, a converter forsupplying the electric motor and a control unit for controlling theconverter that is set up to determine the holding torque necessary forpreventing the vehicle from rolling back.

A converter-friendly control system is achieved according to theinvention by setting up the control unit to control the converter suchthat, as long as a determined motor revolution rate is less than apredetermined first revolution rate limit, a traction torque of thevehicle is limited to a torque that is dependent on the holding torqueand the traction torque is only increased to above the torque limit ifthe motor revolution rate is greater than the first revolution ratelimit.

The description given above of advantageous embodiments of the inventioncontains numerous features that are partly reproduced in the individualdependent claims to form a plurality of combined features. Said featurescan, however, also usefully be considered individually and can becombined to form useful further combinations. In particular, saidfeatures can each be combined individually and in any suitablecombination with the method according to the invention and the deviceaccording to the invention.

The properties, features and advantages of said invention describedabove, as well as the manner in which they are achieved, will becomemore apparent and clearly comprehensible in combination with thefollowing description of the exemplary embodiments, which are describedin detail in connection with the figures. The exemplary embodiments areused to describe the invention and do not limit the invention to thecombination of features presented therein, and also not in relation tofunctional features. Moreover, suitable features of any exemplaryembodiment can also be specifically considered in isolation, removedfrom an exemplary embodiment, introduced into another exemplaryembodiment to extend it and/or combined with any of the claims.

In the figures:

FIG. 1 shows an electrically driven vehicle with three carriages onthree different track segments with different track slope angles,

FIG. 2 shows exemplary time profiles of a traction torque and a braketorque for a drive-off process for which the holding torque is zero,

FIG. 3 shows exemplary time profiles of the traction torque as well asof the brake torque for a drive-off process for which the holding torqueis greater than zero,

FIG. 4 shows an exemplary profile of the traction torque as a functionof a motor revolution rate for the drive-off process of FIG. 2,

FIG. 5 shows an exemplary profile of the traction torque as a functionof the motor revolution rate for the drive-off process of FIG. 3,

FIG. 6 shows an exemplary time profile of a temperature of a bipolartransistor of a converter and

FIG. 7 shows a further exemplary time profile of the temperature of thebipolar transistor for a higher motor revolution rate.

FIG. 1 shows a schematic representation of an electrically drivenvehicle 2 with three carriages 4. The vehicle 2 is a railway vehicle.The right carriage 4 as seen by the observer is implemented as a drivencarriage 4 and the other two carriages 4 are implemented without theirown drives.

The vehicle 2 comprises two electric motors 6 implemented in the form ofalternating current motors that are supplied by means of a converter 8.The converter 8 comprises a bipolar transistor that is not shown in FIG.1 with an isolated gate electrode.

The vehicle 2 also comprises a control system 9 that comprises a controlunit 10 that is set up for controlling a traction torque of the vehicle2. The control unit 10 is in particular configured for controlling thetraction torque by controlling the converter 8. Furthermore, the vehicle2 comprises a motor revolution rate sensor 12 for each of the electricmotors 6 thereof that is configured for measuring a motor revolutionrate of the respective electric motor 6.

The three carriages 4 of the vehicle 2 are each fitted with a pneumaticsuspension system that is not shown in FIG. 1. Moreover, each carriage 4comprises two brake systems 13 that can be controlled by the controlunit 10. Each of the brake systems 13 comprises two brakes, which arenot shown in FIG. 1 for clarity.

Each of the three carriages 4 comprises an accelerometer 14 that isconfigured to measure an acceleration of the carriage 4 that is orientedparallel to a track segment 16. The accelerometers 14 are connected tothe control unit 10 by means of a data line system that is not shown inFIG. 1 and that is configured for transmission of the determinedaccelerations to the control unit 10.

The three carriages 4 of the vehicle 2 are each fitted with a pressuresensor 20 that is configured for measuring a pressure prevailing in thepneumatic suspension system of the respective carriage 4. The pressuresensors 20 are connected by means of the data line system to the controlunit 10 and are configured for the transmission of the determinedpressure to the control unit 10.

In relation to a direction of travel 22, the driven carriage 4 is on atrack segment 16 comprising a downslope—and hence a negative trackgradient angle 24. The forward of the two carriages 4 without adedicated drive is located on a flat track segment 16. The rear of thetwo carriages 4 without a dedicated drive is located on a track segment16 comprising an upslope—and hence a positive track gradient angle 24.The track gradient angle 24 of the track segment 16 with the upslope isgreater in magnitude than the track gradient angle 24 of the tracksegment 16 with the downslope.

In FIG. 1, changes of track gradient angle 24 between the respectivetrack segments 16 are abrupt and the track gradient angle 24 in thedownslope or in the upslope is greater than it may actually be foradhesion railways, which is only used as an illustration.

An acceleration of the respective carriage 4 that is oriented parallelto the track segment 16 on which the carriage 4 is located and that isdependent on the track gradient angle 24 is determined by theaccelerometers 14 of the three carriages 4 at fixed time intervals. Theacceleration is a component of the acceleration due to gravity that isacting as a downhill acceleration. The determined accelerations are thentransmitted to the control unit 10. The motor revolution rates of theelectric motors 6 are determined by means of the two motor revolutionrate sensors 12 in the same time intervals.

The pressures prevailing in the pneumatic suspension systems of therespective carriages 4 are determined by the pressure sensors 20 of thethree carriage 4 and transmitted to the control unit 10. The controlunit 10 calculates a mass of the respective carriage 4 from thetransferred pressures. Furthermore, the control unit 10 calculates thetotal masses of the vehicle 2 from the individual carriage masses.

A carriage holding torque is calculated by the control unit 10 for eachcarriage 4 from the three calculated carriage masses and from thetransmitted accelerations of the three carriages 4, and a holding torquenecessary to prevent the vehicle 2 from rolling back is calculated bysumming all calculated carriage holding torques taking into accounttheir respective signs.

For driving the vehicle 2 off, the control unit 10 controls the brakesystems 13 such that the brakes of the brake systems 13 are released.Consequently, the brake torque produced by the brake systems 13 atwheels 26 of the vehicle 2, starting from an initial value that isgreater than the determined holding torque, is reduced to zero.Moreover, the control unit 10 controls the converter 8 such that whilethe brake torque is being reduced the traction torque acting on thewheels 26 of the vehicle 2 is increased from zero.

FIG. 2 shows a diagram in which exemplary schematic time profiles of atraction torque M_(T) as well as of a brake torque M_(B) during adrive-off process of the railway vehicle described in FIG. 1 areillustrated.

The diagram comprises an ordinate axis and an abscissa axis. A torque Mis plotted on the ordinate axis. A time t is plotted on the abscissaaxis.

Furthermore, the diagram concerns a drive-off process during which therailway vehicle—in contrast to FIG. 1—is on a level track segment, i.e.a determined holding torque M_(F) is zero.

The holding torque is zero during the entire illustrated period of time,since with adhesion railways changes of a track gradient angle takeplace on large length scales in relation to typical carriage lengths ofrailway vehicles, whereas by contrast the railway vehicle in theillustrated period of time only covers a distance of a few carriagelengths.

Initially, the traction torque M_(T) is zero and a brake torque M_(B)produced by the brake systems 13 of the railway vehicle is constant atan initial value, which is greater than zero.

The drive-off process starts at the point in time t₀ at which thecontrol unit 10 activates the brake systems 13 of the vehicle 2 suchthat the brake systems 13 release their brakes. From the point in timet₀ the brake torque M_(B) decreases starting from the initial value. Forsimplicity, in FIG. 2 the brake torque M_(B) decreases at a constantrate. The rate at which the brake torque M_(B) decreases can alsofluctuate with time however.

A first point in time t₂, at which the brake torque M_(B) will reduce tozero, is precalculated by the control unit 10. A second point in time t₁is calculated based on the first point in time t₂. Said second point intime t₁ is characterized in that the traction torque M_(T), increased ata maximum allowed rate, reaches a torque limit M_(G) at the first pointin time t₂ if the traction torque M_(T) at the point in time t₁ isincreased starting from zero.

From the point in time t₁ the traction torque M_(T) is increased by thecontrol unit 10 starting from zero. As precalculated, the brake torqueM_(B) decreases to zero at the point in time t₂ and the traction torqueM_(T) reaches the torque limit M_(G) at the point in time t₂. Once thetraction torque M_(T) is greater than the brake torque M_(B) and inaddition frictional resistances in bearings of the vehicle 2 areovercome, i.e. between the point in time t₁ and the point in time t₂,the railway vehicle starts to drive in the direction of travel and amotor revolution rate of the two electric motors 6 increases starting atzero.

The torque limit M_(G) is set by the control unit 10 such that a maximumtemperature of the bipolar transistor that is reached during aconducting phase of the bipolar transistor of the converter 8 remainsbelow a specified temperature value. In the present case, the torquelimit M_(G) is equal to 0.25 times the maximum of the traction valueM_(end) that can be set by the control unit 10.

The traction torque M_(T) is increased at an average rate, which isequal to a maximum allowed rate, up to the point in time t₂, whereinsaid maximum allowed rate is less than a technically possible maximumrate at which the traction torque M_(T) can increase. The maximumallowed rate is rather a rate limited for passenger comfort reasons, inparticular in relation to avoiding sudden jolts, as well as forprotecting a drive train of the vehicle 2.

The average rate at which the traction torque M_(T) is increased isgreater in magnitude than the rate at which the brake torque M_(B)decreases.

At the start of the increase in the traction torque M_(T) at the pointin time t₁, a small, practically instantaneous jump in the tractiontorque M_(T) of height of approx. 5% of the maximum of the tractionvalue M_(end) that can be set by the control unit 10 takes place and isused to increase the traction torque M_(T) more rapidly. In the event ofa jump in the traction torque M_(T) of such a small height, owing todamping by a suspension system of the vehicle 2, neither detectablejolts nor significant wear on the drive train of the vehicle 2 occurs.

After exceeding a minimum torque M_(min), the traction torque M_(T) isheld above the minimum torque M_(min) by the control unit 10 for therest of the drive-off process, wherein the minimum torque M_(min) in thepresent case equals 0.15 times the maximum of the traction value M_(end)that can be set by the control unit 10.

From the point in time t₂, at which the traction torque M_(T) is of thesame magnitude as the torque limit M_(G), the traction torque M_(T) isheld constant by the control unit 10 at the torque limit M_(G) until themotor revolution rate reaches a predetermined first revolution ratelimit.

The predetermined first revolution rate limit is reached at the point intime t₃. From said point in time the traction torque M_(T) is increasedby the control unit 10, in particular in proportion to the motorrevolution rate, until the motor revolution rate reaches a predeterminedsecond revolution rate limit.

The predetermined second revolution rate limit is reached at the pointin time t₄. At said point in time, the traction torque M_(T) takes themaximum traction value M_(end) that can be set by the control unit 10.The ratio of the second revolution rate limit to the first revolutionrate limit equals the ratio of the maximum traction value M_(end) thatcan be set by the control unit 10 to the torque limit M_(G).

From the point in time t₄, the traction torque M_(T) is held constant atthe maximum traction value M_(end) that can be set by the control unit10 until reaching maximum motor power at point in time t₅.

The following descriptions of the other figures are essentially eachlimited to the differences from the immediately previously describedfigure.

FIG. 3 shows a diagram in which further exemplary time profiles of thetraction torque M_(T) as well as of the brake torque M_(B) areschematically illustrated. The diagram concerns a drive-off processduring which the railway vehicle is on an upslope, thus the determinedholding torque M_(F) is greater than zero.

For simple comparability of FIG. 3 and FIG. 2, the scale of the ordinateaxes and the abscissa axes is the same in both figures.

The initial value of the brake torque M_(B), which is exactly the sameas in FIG. 2, is greater than the determined holding torque M_(F). Inthe present case, the magnitude of the holding torque M_(F) is approx.0.5 times the maximum of the traction value M_(end) that can be set bythe control unit 10. The torque limit M_(G) is equal to 1.25 times theholding torque M_(F) and the minimum torque M_(min) is equal to 1.1times the holding torque M_(F). The increase in traction torque M_(T)starts not from the precalculated point in time t₁ but from a precedingpoint in time t₁″. The average rate at which the traction torque M_(T)is increased from the point in time t₁′ is in this case selected asequal in magnitude to the rate at which the brake torque M_(B)decreases, so that a magnitude of a vector sum of the brake torque M_(B)and the traction torque M_(T) remains approximately constant from thepoint in time t₁″ to the point in time t₂.

An increase of the traction torque M_(T) at the maximum allowed ratefrom the precalculated point in time t₁ that is analogous to FIG. 2would result in the brake torque M_(B) already being below the torquelimit M_(G) and possibly even below the holding torque M_(F) immediatelybefore the point in time t₁, which is still before the traction torqueM_(T) has built up. As a result, rolling back of the vehicle 2 could notbe safely prevented.

The railway vehicle starts to drive in the direction of travel and themotor revolution rate of the electric motors 6 increases once adifference of the traction torque M_(T) and the holding torque M_(F) isgreater than the brake torque M_(B) and frictional resistances inbearings of the vehicle 2 are also overcome, i.e. between the point intime t₁″ and the point in time t₂.

Because in the present case the torque limit M_(G) is greater than inFIG. 2, and the average rate with which the traction torque M_(T) isincreased until reaching the torque limit M_(G) is also less than inFIG. 2, in the present case there is a period of time to reach thetorque limit M_(G) from the start of the increase of the traction torqueM_(T) from the value zero that is longer than in FIG. 2.

As can be seen from a comparison of FIG. 3 and FIG. 2, a period of timein which the traction torque M_(T) is held at the torque limit M_(G) islonger in FIG. 3 than in FIG. 2. This is because in the present case thetorque limit M_(G) is greater than in FIG. 2 and consequently longerduration limiting of the traction torque M_(T) takes place in order toprotect the converter 8.

Furthermore, it can be seen from the comparison of FIG. 3 and FIG. 2that a period of time in which a traction torque M_(T) increase that isproportional to the motor revolution rate takes place is shorter in FIG.3 than in FIG. 2, which is because the increase proportional to themotor revolution rate starts at a higher traction torque M_(T).

FIG. 4 shows a diagram in which an exemplary profile of the tractiontorque M_(T) is illustrated schematically as a function of the motorrevolution rate n. The diagram concerns the time profile of the tractiontorque M_(T) that is illustrated in FIG. 2, as well as the drive-offsituation that is described in connection with FIG. 2.

The diagram comprises an ordinate axis and an abscissa axis. A torque Mis plotted on the ordinate axis. The motor revolution rate n is plottedon the abscissa axis.

While the motor revolution rate n is less than the predetermined firstrevolution rate limit n₁, the traction torque M_(T) is set by thecontrol unit 10 to the torque limit M_(G), which is 0.25 times themaximum of the traction value M_(end) that can be set by the controlunit 10. As described in connection with FIG. 2, the railway vehiclestarts to drive in the direction of travel once the traction torqueM_(T) is greater than the brake torque M_(B) and frictional resistancesin bearings of the vehicle 2 are also overcome, i.e. still before thepoint in time t₂ at which the traction torque M_(T) is equal to thetorque limit M_(G). As traction has already built up before the railwayvehicle starts to drive in the direction of travel, with the motorrevolution rate at zero the traction torque M_(T) is already at a valuegreater than zero, but less than the torque limit M_(G). Because themotor revolution rate n increases in proportion to the speed of thevehicle 2, the traction torque M_(T) increases with rising revolutionrate n, in particular linearly with revolution rate n, until reachingthe torque limit M_(G) at the point in time t₂.

After reaching the torque limit M_(G), the traction torque M_(T) is heldconstant by the control unit 10 at the torque limit M_(G) until themotor revolution rate n reaches the predetermined first revolution ratelimit n₁ at the point in time t₃. Once the motor revolution rate n hasexceeded the predetermined first revolution rate limit n₁ and while themotor revolution rate n is less than the predetermined second revolutionrate limit n₂, the traction torque M_(T) is increased by the controlunit 10 in proportion to the motor revolution rate n.

On reaching the second revolution rate limit n₂ at the point in time t₄,the traction torque M_(T) equals the maximum of the traction valueM_(end) that can be set by the control unit 10. As long as the maximummotor power has not yet been reached, the traction torque M_(T) is heldconstant at the traction value M_(end) that can be set by the controlunit 10. From the point in time t₅ at which the maximum motor power isreached, the traction torque M_(T) is conversely reduced in proportionto the motor revolution rate n, whereas the maximum motor power ismaintained.

FIG. 5 shows a diagram in which a further exemplary profile of thetraction torque M_(T) as a function of the motor revolution rate n isschematically illustrated. The diagram concerns the time profile of thetraction torque M_(T) that is illustrated in FIG. 3 as well as thedrive-off situation that is described in connection with FIG. 3.

For simple comparability of FIG. 5 and FIG. 4, the scale of the ordinateaxes and the abscissa axes is the same in both figures.

With the motor revolution rate at zero, the traction torque M_(T) isalready at a value that is greater than the holding torque M_(F), butless than the torque limit M_(G).

The torque limit M_(G), to which the traction torque is set by thecontrol unit 10, equals 1.25 times the determined holding torque M_(F),wherein the holding torque M_(F) is approx. 0.5 times the maximum of thetraction value M_(end) that can be set by the control unit 10. Thetorque limit M_(G) in the present case is thus larger than in FIG. 4.Accordingly, the predetermined first revolution rate limit n₁, untilreaching which the traction torque M_(T) is held at the torque limitM_(G), is set by the control unit 10 to a larger value than in FIG. 4 toprotect the converter 8.

The predetermined second revolution rate limit n₂, until reaching whichthe traction torque M_(T) is increased in proportion to the motorrevolution rate n after exceeding the predetermined first revolutionrate limit n₁, by contrast is set by the control unit 10 to the samevalue as in FIG. 4.

The torque limit to which the traction torque is limited while the motorrevolution rate n is less than the predetermined first revolution ratelimit n₁, can be adjusted by the driver in stages, in particular inthree setting steps.

The first setting step is set by default. Selection of the second orthird setting step is limited to the existence of an operationalexception situation and must be enabled by the driver by operating anunlocking lever or an unlocking switch.

In the first setting step the limiting of the traction torque is carriedout as previously described, i.e. the limiting of the traction torqueM_(T) to the torque limit M_(G) described in connection with FIGS. 2through 5 concerns the first setting step. In the second setting stepthe torque limit is set such that a difference between the maximum ofthe traction value M_(end) that can be set by the control unit 10 andthe torque limit is halved compared to the corresponding difference inthe first setting step.

In the third step by contrast no traction torque limiting occurs.

If there is an operational exception situation, faster driving off ofthe railway vehicle can be achieved by selecting the second or the thirdsetting step.

In FIG. 2 an exemplary time profile of the traction torque M_(T)′ forselection of the second setting step is illustrated using a dashed line.

In the second setting step the torque limit M_(G)′ is set to a valueamounting to approx. 0.62 times the maximum of the traction valueM_(end) that can be set by the control unit 10. As a result, thedifference between the maximum of the traction value M_(end) that can beset by the control unit 10 and the torque limit M_(G)′ is halvedcompared to the difference between the maximum of the traction valueM_(end) that can be set by the control unit 10 and the torque limitM_(G) in the first setting step.

Because the torque limit M_(G)′ in the second setting step is greaterthan the torque limit M_(G) in the first setting step, the precalculatedpoint in time t₁′ in the second setting step is before the precalculatedpoint in time t₁ in the first setting step. The traction torque M_(T)′that is increased from the point in time t₁′ with a maximum allowed ratereaches the torque limit M_(G)′ at the point in time t₂.

Up to the point in time t₃′ at which the motor revolution rate n reachesthe first revolution rate limit n₁, the traction torque M_(T)′ is heldconstant by the control unit 10 at the torque limit M_(G)′. From thepoint in time t₃′ an increase of the traction torque M_(T) takes placein proportion to the motor revolution rate n until the traction torqueM_(T)′ reaches the maximum of the traction value M_(end) that can be setby the control unit 10 at the point in time t₄′. From that point untilthe maximum motor power is reached at point in time t₅, the tractiontorque M_(T)′ remains constant at the maximum of the traction valueM_(end) that can be set by the control unit 10.

In the second setting step, a period of time in which the tractiontorque M_(T)′ is held at the torque limit M_(G)′ is longer than in thefirst setting step. This is because the torque limit M_(G)′ is greaterin the second setting step than in the first setting step, andconsequently longer duration limiting of the traction torque M_(T)′occurs in order to protect the converter 8.

Furthermore, in the second setting step a period of time in which theincrease of the traction torque M_(T)′ in proportion to the motorrevolution rate n occurs is shorter than in the first setting step,because the increase in proportion to the motor revolution rate n startsat a higher traction torque M_(T)′ than in the first setting step.

The variation against time of the traction torque M_(T)′ in the secondsetting step described in connection with FIG. 2 can be transferredanalogously to FIG. 3.

FIG. 6 shows a diagram in which an exemplary time profile of atemperature of the bipolar transistor of the railway vehicle that isdescribed in FIG. 1 is illustrated schematically. The diagram comprisesan ordinate axis and an abscissa axis. A temperature T is plotted on theordinate axis. A time t is plotted on the abscissa axis.

The illustrated temperature is the temperature at a contact surface ofthe bipolar transistor on which a bonding wire is soldered or weldedonto the bipolar transistor. Said temperature can for example be thetemperature to which the control unit 10 refers when controlling thetraction torque M_(T).

Furthermore, the illustrated temperature profile relates to a period oftime in which a motor revolution rate n of the two electric motors 6 isless than the predetermined first revolution rate limit n₁, and hence afrequency of the output voltage produced by the converter 8 is low. Theillustrated period of time is so short that the motor revolution rate nin said period of time is considered to be approximately constant.

During a conducting phase of the bipolar transistor, the bipolartransistor heats up and the temperature at the contact surfaceincreases. Accordingly, the bipolar transistor cools down during anon-conducting phase of the bipolar transistor and the temperature atthe contact surface decreases.

A periodic fluctuation of the temperature between a minimum temperatureand a maximum temperature is shown in FIG. 6. The maximum temperature isa maximum temperature T_(max) reached at the contact surface during abipolar transistor conducting phase. Said temperature is reached at theend of the conducting phase. The minimum temperature is a minimumtemperature T_(min) at the contact surface reached during anon-conducting phase of the bipolar transistor. Said temperature isreached at the end of the non-conducting phase. Consequently, the valueof the minimum temperature T_(min) reached at the contact surfacedepends inter alia on a duration of the non-conducting phase of thebipolar transistor. Accordingly, the value of the maximum temperatureT_(max) reached at the contact surface depends inter alia on a durationof the conducting phase of the bipolar transistor.

The temperature profile is illustrated in a simplified form and is onlyintended to illustrate a relationship between the motor revolution raten dependent on the frequency of the output voltage and the maximumtemperature T_(max) reached at the contact surface during a conductingphase of the bipolar transistor. For the illustration of the temperatureprofile it was assumed that an average temperature at the contactsurface—in relation to a time average of the temperature over aplurality of temperature periods—has been set to a constant value anddoes not increase with time t.

FIG. 7 shows a diagram in which a further exemplary time profile of thetemperature of the bipolar transistor of the converter 8 is illustrated.The illustrated temperature profile relates to a period of time in whichthe motor revolution rate n is greater than the first predeterminedrevolution rate limit n₁, and thus the frequency of the output voltageproduced by the converter 8 is higher than in FIG. 6.

For simple comparability of FIG. 6 and FIG. 7, the scale of the ordinateaxes and the abscissa axes is the same in both figures.

The duration of a conducting phase of the bipolar transistor isinversely proportional to the frequency of the output voltage producedby the converter 8. Therefore, the duration of a conducting phase of thebipolar transistor is shorter the higher is the motor revolution rate n.Accordingly, the bipolar transistor heats up for a shorter period at ahigher motor revolution rate n. As a result, the maximum temperatureT_(max) reached during a conducting phase of the bipolar transistor atthe contact surface can be lower for a higher motor revolution rate nthan for a lower motor revolution rate n—even though a rate at which thetemperature rises at the higher motor revolution rate n can be higher,for example since the traction torque M_(T) is greater.

This fact is apparent when comparing FIG. 6 and FIG. 7. Thus in FIG. 7the rate at which the temperature rises is greater than in FIG. 6. Butbecause the duration of a conducting phase in FIG. 7 is shorter than inFIG. 6, the maximum temperature T_(max) reached at the contact surfaceduring a conducting phase of the bipolar transistor is lower in FIG. 7than in FIG. 6.

At a higher motor revolution rate n, in addition the duration of anon-conducting phase of the bipolar transistor is also shorter.Therefore, the bipolar transistor cools down for less time at a highermotor revolution rate n and the minimum temperature T_(min) reached atthe contact surface during a non-conducting phase of the bipolartransistor can be greater for a higher motor revolution rate n than fora lower motor revolution rate n. For simplicity it can be assumed thatthe duration of a non-conducting phase of the bipolar transistor in FIG.7 is so long that an increase in the minimum temperature T_(min) that isreached in FIG. 7 compared to the minimum temperature T_(min) that isreached in FIG. 6 is negligible.

Although the invention has been illustrated and described in detailusing the preferred exemplary embodiment, the invention is not limitedby the disclosed example and other variations can be derived therefromby a person skilled in the art without departing from the scope ofprotection of the invention.

1-15. (canceled)
 16. A method for controlling a drive-off process of anelectrically driven vehicle, with which a holding torque necessary forpreventing the electrically driven vehicle from rolling back isdetermined, which comprises the steps of: limiting a traction torque viaa control unit of the electrically driven vehicle to a torque limitdependent on the holding torque if a determined motor revolution rate isless than a predetermined first revolution rate limit; and raising thetraction torque above the torque limit by the control unit if thedetermined motor revolution rate exceeds the predetermined firstrevolution rate limit.
 17. The method according to claim 16, wherein thetorque limit is less than a maximum of 1.3 times the holding torque and0.3 times a maximum traction value that can be set by the control unit.18. The method according to claim 16, wherein the torque limit exceedsthe maximum of 1.2 times the holding torque and 0.2 times the maximumtraction value that can be set by the control unit.
 19. The methodaccording to claim 16, which further comprises increasing the tractiontorque linearly in relation to the determined motor revolution rate bythe control unit until a predetermined second revolution rate limit isreached once the determined motor revolution rate is greater than thepredetermined first revolution rate limit.
 20. The method according toclaim 16, which further comprises setting the predetermined firstrevolution rate limit via the control unit depending on the holdingtorque.
 21. The method according to claim 19, wherein a ratio of thepredetermined second revolution rate limit to the predetermined firstrevolution rate limit equals a ratio of a maximum of a traction valuethat can be set by the control unit to the torque limit.
 22. The methodaccording to claim 16, which further comprises: determining for eachcarriage of the electrically driven vehicle a respective carriage massand a carriage gradient parameter dependent on a track gradient angle;calculating a carriage holding torque from the respective carriage massand the carriage gradient parameter; and calculating the holding torqueof the electrically driven vehicle by summing all determined carriageholding torques.
 23. The method according to claim 16, wherein a brakeis released by the control unit for driving off and a rise of thetraction torque takes place in dependence on a reducing brake torque.24. The method according to claim 16, wherein a brake is released by thecontrol unit for driving off before the traction torque is increased bythe control unit starting from zero.
 25. The method according to claim16, wherein a brake is released by the control unit for driving off andthe traction torque is controlled by the control unit so that thetraction torque first reaches the torque limit when a brake torquereaches zero.
 26. The method according to claim 16, which furthercomprises increasing the traction torque via the control unit so that asum of the traction torque and a brake torque remains constant.
 27. Themethod according to claim 16, which further comprises: precalculating afirst point in time at which a brake torque will have fallen to zero;and calculating a preceding second point in time from the first point intime, so that, rising at a maximum allowed rate from zero at thepreceding second point in time, the traction torque reaches the torquelimit at the first point in time.
 28. The method according to claim 16,wherein if the determined motor revolution rate falls below thepredetermined first revolution rate limit the traction torque is held bythe control unit at the torque limit while the determined motorrevolution rate is less than the predetermined first revolution ratelimit, and the traction torque is increased by the control unit to abovethe torque limit once the determined motor revolution rate exceeds thepredetermined first revolution rate limit.
 29. The method according toclaim 16, wherein the traction torque is a negative traction torque forelectrical braking.
 30. The method according to claim 16, which furthercomprises increasing the traction torque via the control unit so that asum of the traction torque and a brake torque is equal to the torquelimit.
 31. A control system for an electrically driven vehicle having atleast one electric motor and a converter for supplying the electricmotor, the control system comprising: a control unit for controlling theconverter, said control unit configured to determine a holding torquethat is necessary to prevent rolling back of the electrically drivenvehicle, said control unit further configured to control the converterso that a determined motor revolution rate is less than a predeterminedfirst revolution rate limit, and a traction torque of the vehicle islimited to a torque limit that is dependent on the holding torque andthe traction torque is only then raised above the torque limit if thedetermined motor revolution rate is greater than the predetermined firstrevolution rate limit.